There are known semiconductor devices with a thin film transistor (hereinafter referred to as TFT) formed on an insulating substrate made of glass, etc., such as an active-matrix type liquid crystal display device wherein a TFT is used as a switching element, an image sensor, etc.
In the TFT adopted in the described devices, generally, an amorphous silicon film, or a polysilicon film having crystallinity is used as a semiconductor film made of a thin-film shaped silicon. The TFT adopting the described silicon film, however, has the problem of changes in characteristics such as an increase in OFF-state current, a change in threshold voltage, etc., when a light beam is incident on a semiconductor film, which would result in lower reliability of the TFT.
To eliminate the described problem, a technique of forming a light-shielding film in a channel region has been adopted. As shown in FIG. 3(a) and FIG. 3(b), a conventional TFT having the light-shielding film in a channel region includes an insulating substrate 1, a light-shielding film 2, an insulating film 3, a semiconductor film 4, a gate insulating film 6, a gate electrode 7, an interlayer insulating film 8, a contact hall 9, a source electrode 10 and a drain electrode 11.
The conventional process of manufacturing the TFT wherein the light-shielding film is formed in the channel region will be explained in reference to FIG. 4(a) through FIG. 4(d).
First, a metallic film 2 of metal such as Ta, etc., is formed with a thickness of 100 nm as a light-shielding film on the insulating substrate 1 of glass, etc. Thereafter, a photoresist is deposited thereon, and the photoresist is patterned to form a photoresist 12a in a predetermined shape so as to entirely cover at least the channel region.
Next, an etching of the metallic film 2 is performed using the patterned photoresist 12a as a mask. After completing the etching process, the photoresist 12a is removed. As a result, the metallic film 2 is patterned into a predetermined shape so as to entirely cover the channel region. Thereafter, the insulating film 3, and the semiconductor film 4 wherein the channel region, the drain region and the source region are formed are deposited in this order on the entire surface of the insulating substrate 1 and the metallic film 2.
Further, a photoresist is deposited on the semiconductor film 4, and the photoresist 12b is patterned into a predetermined shape as shown in FIG. 4(b). Next, as shown in FIG. 4(c), the semiconductor film 4 is patterned into a predetermined shape using the photoresist 12b as a mask, and thereafter, the photoreceptor 12b is removed.
Then, as shown in FIG. 4(d), the gate insulating film 6 and the gate electrode 7 are formed, and the interlayer insulating film 8 is formed. Further, a contact hall 9, a source electrode 10 and a drain electrode 11 are formed.
However, the described conventional method of forming the light-shielding film requires significantly increased number of processes including the process of forming a resist pattern, an etching process of the light-shielding film, etc., compared with the case where the light-shielding film is not formed.
Besides, the characteristics of the TFT are determined by a current flowing in the channel region. Therefore, in order to prevent irregularities in characteristics of the TFT, it is required to prevent at least the channel region from having a light beam incident thereon. Therefore, it is necessary to form the light-shielding film so as to cover at least the channel region. However, the conventional method does not provide a solution to solve the problem of positioning error occurred when forming the resist pattern, thereby failing to form the light-shielding film on a position as designed with accuracy.
In consideration of the described positioning error, it is required that the light-shielding film be designed to be larger than the channel region of the semiconductor film. Therefore, as shown in FIG. 3(b), the light-shielding film has a larger TFT portion for the area protruded from the channel region, and an aperture ratio representing a ratio of area subject to display would be lowered.
Namely, the aperture ratio indicates a ratio of the display region in each pixel, i.e., a ratio of a region which is not shielded by the TFT portion to the whole pixel size, and the greater is the size of the TFT portion which is used as a switching element in each pixel, the more reduced is the display region, thereby lowering the aperture ratio.
Besides, according to the described conventional method, as a difference in level arises on the surface of the semiconductor film, such problem that the semiconductor film is disconnected at a portion where the difference in level is formed occurs.